1. Field of the Invention
The present invention relates, generally, to light beam steering mechanisms, and more particularly, to an apparatus and method for steering a light beam which utilizes the relative positioning of adjacently disposed semi-spherical windows to provide the desired steering.
2. Description of the Prior Art
U.S. Pat. No. 4,289,379 issued to G. Michelet discloses a variable focal length lens including a fluid medium within the optical path. The device uses a piezoelectric element which modifies the curvature of the lens in response to an input voltage.
U.S. Pat. No. 4,614,405 issued to W. M. Brandenberg discloses a wide-angle laser window adapted to be mounted to an aircraft. It contains a first transmissive window and second transmissive window pivotally mounted relative to the first window. Flexible seal means join the first and second windows to define a fluid cell therebetween containing a fluid. The first window, second window and fluid have indices of refraction which are substantially matched for generally undegraded transmission of the light beam.
Although Brandenberg teaches optical steering using fluid between the two windows, precision optical steering cannot be achieved with this device. Time-dependent thermal variations between the glass and fluid will cause index variations resulting in time-dependent beam steering.
For example, if the windows were made of synthetic fused silica with an index of refraction typically equal to 1.4584 at 589.3 nm and index matched with a standard fused silica matching fluid of index of refraction typically equal to 1.4587 at 589.3 nm, the difference in index is only 3.times.10.sup.-4 which is constant with time. However, the rate of change of the index of refraction due to temperature change (dn/dt) is 1.28.times.10.sup.-5 /.degree. C. for fused silica for the temperature range 0.degree. to 700.degree. C. compared to -3.86.times.10.sup.-4 between 15.degree. to 35.degree. C. for the index matching fluid. This difference in thermal index of refraction between the glass and fluid will steer the beam variable amounts. This temperature sensitive beam steering is a major disadvantage of the Brandenberg patent for precision beam steering.
For beam steering with an angular precision requirement of one milliradian or less with a nominal beam deviation of 0.1 radian, the temperature differential between the window and the fluid would have to be less than 0.03.degree. C. The use of fluid between the windows is undesirable for precision beam steering where the angle of incidence is changing on the second window.
Of a particular concern in the Brandenberg patent is the generation of pressure waves in the index matching fluid as the beam steering angle is changed. These pressure waves will lead to small index of refraction variations affecting both beam quality and angular beam jitter.